This invention relates generally to magnetic recording media, and more particularly to thermally stable high density media.
Conventional media for horizontal or in-plane magnetic recording, such as the rigid magnetic recording disks in hard disk drives, typically use a granular ferromagnetic material, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the magnetic layer for the recording medium. Each magnetized domain in the magnetic layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the xe2x80x9cbitsxe2x80x9d of the recorded data. IBM""s U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (Hc) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hc. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the layer will exhibit increasing magnetic decay, which has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions. One approach to the solution of this problem is to move to a higher anisotropy material (higher Ku). However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too great to be written by a conventional recording head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
Magnetic recording media with high intrinsic SNR (low intrinsic media noise) is desirable because it is well known in metal alloy media, such as CoPt alloys, that the intrinsic media noise increases with increasing linear recording density. Media noise arises from irregularities in the magnetic transitions and results in random shifts of the readback signal peaks. These random shifts are referred to as xe2x80x9cpeak jitterxe2x80x9d or xe2x80x9ctime jitterxe2x80x9d. Thus higher media noise leads to higher bit error rates. It is therefore desirable to develop a thin film metal alloy magnetic media that generates noise below a maximum acceptable level so that data can be recorded at maximum linear density. It is known that substantially improved SNR can be achieved by replacing a single magnetic layer with a laminated magnetic layer of two (or more) separate magnetic layers that are spaced apart by an nonmagnetic spacer layer. This discovery was made by S. E. Lambert, et al., xe2x80x9cReduction of Media Noise in Thin Film Metal Media by Laminationxe2x80x9d, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2706-2709, and subsequently patented in IBM""s U.S. Pat. No. 5,051,288. The reduction in media noise by lamination is believed due to a decoupling of the magnetic interaction or exchange coupling between the magnetic layers in the laminate. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer layer thicknesses, from 5 to 400 A, that result in the best decoupling of the magnetic layers, and thus the lowest media noise. This work has been reported in papers by E. S. Murdock, et al., xe2x80x9cNoise Properties of Multilayered Co-Alloy Magnetic Recording Mediaxe2x80x9d, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2700-2705; A. Murayama, et al., xe2x80x9cInterlayer Exchange Coupling in Co/Cr/Co Double-Layered Recording Films Studied by Spin-Wave Brillouin Scatteringxe2x80x9d, IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, pp. 5064-5066; and S. E. Lambert, et al., xe2x80x9cLaminated Media Noise for High Density Recordingxe2x80x9d, IEEE Transactions on Magnetics, Vol. 29, No. 1, January 1993, pp. 223-229. U.S. Pat. No. 5,462,796 and the related paper by E. Teng et al., xe2x80x9cFlash Chromium Interlayer for High Performance Disks with Superior Noise and Coercivity Squarenessxe2x80x9d, IEEE Transactions on Magnetics, Vol. 29, No. 6, November 1993, pp. 3679-3681, describe a laminated low-noise disk that uses a discontinuous Cr film that is thick enough to reduce the exchange coupling between the two magnetic layers in the laminate but is so thin that the two magnetic layers are not physically separated. However, in conventional laminated media, because the spaced-apart magnetic layers have their magnetic moments oriented parallel in the two remanent magnetic states (zero applied magnetic field), the Mrt of the laminated media is the sum of the Mrt of each of the individual magnetic layers and thus there is no reduction in Mrt and no improvement in thermal stability.
IBM""s U.S. Pat. No. 6,280,813 describes antiferromagnetically coupled (AFC) media, wherein the magnetic recording layer is at least two ferromagnetic films exchange coupled together antiferromagnetically across a nonferromagnetic spacer film (also called the antiferromagnetic coupling film). The antiferromagnetic exchange coupling, which is believed to originate from the Ruderman-Kittel-Kasuya-Yoshida (RKKY) coupling typically found in Co/Ru/Co multilayers, produces an exchange field (Haf) that is greater than the coercive field of the lower ferromagnetic film. As a result the two ferromagnetic films have their moments oriented antiparallel in the two remanent magnetic states (zero applied magnetic field). Because the magnetic moments are oriented antiparallel, the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the top ferromagnetic film remain unchanged since its thickness is unchanged. The exchange coupling oscillates from antiferromagnetic to ferromagnetic with decreasing coupling strength as the thickness of the spacer film increases, as described by Parkin et al. in xe2x80x9cOscillations in Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/Ru, Co/Cr and Fe/Crxe2x80x9d, Phys. Rev. Lett., Vol. 64, p. 2034 (1990).
Antiferromagnetic exchange coupling of ferromagnetic films was previously described with respect to spin-valve type giant magnetoresistance (GMR) recording heads and magnetic tunnel junction (MTJ) devices as a way to design continuous magnetized antiferromagnetically exchange coupled films whose magnetic moments are rigidly coupled together antiparallel during operation. These types of structures are described, for example, in IBM""s U.S. Pat. Nos. 5,408,377 and 5,465,185. IBM""s U.S. Pat. No. 6,166,948 describes an MTJ device with a continuous magnetized antiparallel-coupled ferromagnetic structure wherein antiferromagnetic exchange coupling is deliberately avoided. Instead, the two ferromagnetic films making up the structure are magnetostatically coupled at their ends by the dipole fields that emanate from the ends of the films.
In AFC media the antiferromagnetic coupling film must be selected from a list of known materials and must be of a special thickness to give rise to the exchange coupling between the two ferromagnetic films, which limits the material selection and manufacturing options in the design of the media structure.
What is needed is magnetic recording media that will support very high density recording while retaining good thermal stability and SNR and providing more options in material selection and manufacturing.
The invention is an antiparallel-coupled (APC) medium for horizontal magnetic recording wherein the magnetic recording layer comprises two ferromagnetic films with different Mrt values that are coupled antiparallel predominantly by dipole fields across a nonferromagnetic spacer film. The material compositions and thicknesses of the ferromagnetic films and the nonferromagnetic spacer film are selected so that the dipole field (Hd) from the grains of the higher-Mrt ferromagnetic film predominates over any antiferromagnetic exchange field (Haf) and is greater than the coercive field of the lower-Mrt ferromagnetic film. As a result, the magnetizations of the two ferromagnetic films will be antiparallel in the two remanent magnetic states. The net magnetic moment of the recording layer has two possible orientations, resulting in the two stable remanent magnetic states. In the APC medium, the net remanent magnetization-thickness product (Mrt) of the recording layer is reduced because it is the difference in the Mrt values of the two ferromagnetic films, and this reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the higher-Mrt ferromagnetic film remain unchanged because the thickness is unchanged.
The spacer film in the APC medium can be a material that is not known to cause any substantial antiferromagnetic exchange coupling, in which case the thickness of the spacer film is less critical and can even be made very thin. The spacer film can also be a material that is known to cause antiferromagnetic exchange coupling, such as Ru or Cr, but can be made thick enough to avoid oscillatory coupling peaks so that any substantial antiferromagnetic exchange coupling is prevented and dipole coupling predominates. In one embodiment of the APC medium, a 6 xc3x85 RuCr spacer film results in a dipole field twice as large as the antiferromagnetic exchange coupling field.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.